There is known an optical sensor for determining the quantity of particles in a sample of fluid. The sensor comprises a flow-through cell with two windows. A liquid envelope flows through the cell, and a sample of fluid to be analyzed is introduced into the liquid envelope which separates the samples from particles on the cell walls and accurately directs particles contained in the sample to the focus of a light beam admitted through the windows of the cell.
There is further known a device for granulometric analysis of particles contained in fluids, comprising a flow-through chamber of an analyzer, having an inlet and an outlet. The device further includes a lighting means arranged outside the flow-through chamber, opposite a window which is transparent to light and provided in the wall of the chamber. The lighting means is intended to direct a convergent light beam into the chamber. The device further includes a light-sensitive means to receive light reflected from particles contained in the fluid subjected to granulometric analysis. The light-sensitive means is arranged outside the chamber, opposite a second window provided in the wall of the chamber, which second window is transparent to reflected light. The optical axis of the light-sensitive means extends at a perpendicular to the optical axis of the lighting means and intersects it inside the chamber. The inlet of the chamber communicates with a tubular nozzle arranged in the center of the chamber. Spaced around the tubular nozzle are pipes communicating with a source of pure fluid and intended to produce a liquid envelope that encompasses the flow of the fluid being investigated.
The optical properties of the liquid envelope must be identical with those of the fluid being investigated. This requirement applies to each of the above-mentioned optical sensor and device for granulometric analysis of particles contained in fluids. Identical optical properties of the liquid envelope and fluid under investigation can best be attained when both have the same composition, provided that the fluid used to produce the liquid envelope is pure, i.e. free from all foreign particles with a grain size in excess of the sensitivity threshold of the light-sensitive means. This requirement makes it imperative that the device for granulometric analysis should include such units as a tank for fluid, a pump and a high-performance purifier of fluid. In addition, provision must be made for a special means for equalizing the velocities of the liquid envelope and the fluid being investigated in order to prevent mixing of these flows in the zone of intersection of the fluid being investigated and the light beam, i.e. prevent a transfer of particles from the fluid being investigated to the liquid envelope.
It must further be remembered that a device for granulometric analysis is normally used to analyze different types of fluid. The liquid envelope must be changed each time a different fluid is to be investigated so that its optical properties should correspond to those of the fluid. Such changes significantly affect the rate of analysis and increase the costs involved.